1. Field of the Invention
This invention relates to transmission space diversity, and more particularly to a transmission space diversity transmission method and a transmission space diversity apparatus suitable for use with a duplex communication system for different transmission and reception frequency signals.
2. Description of the Related Art
In a radio communication system, a diversity system is adopted in order to prevent deterioration in communication quality caused by an obstacle to radio wave propagation between transmitting and receiving stations.
Particularly, for a propagation path in which an obstacle to radio wave propagation is caused by the influence of shadowing caused by an obstacle to an antenna and multi-path fading caused by composite attenuation by a direct wave and a reflected wave or the like, a reception space diversity system is usually utilized as a countermeasure to the obstacle to radio wave propagation.
However, in a time division multiple access (TDMA) radio communication system wherein a plurality of terminal stations communicate with one base station like a mobile communication system, the reception space diversity system cannot sometimes be utilized from a demand for miniaturization or cost lowering of a terminal station apparatus. In such an instance, a transmission space diversity system is adopted wherein a plurality of antennas are provided on the transmission side and one of the antennas with which a terminal station can receive a radio wave with a comparatively high quality is selected to transmit a radio wave.
FIG. 1 shows a conventional example wherein such a transmission space diversity system as described above is applied to a TDMA communication system of mobile communication (Japanese Patent Laid-Open Application No. Heisei 5-29992).
The circuit shown in FIG. 1 includes two antennas 21 and 22 each for use during transmission and reception, receivers 23 and 24 corresponding to antennas 21 and 22, respectively, error detection circuits 29 and 30 for received signals, error bit comparison circuit 26, reception output selection switch 28, transmitter 25, and change-over switch 27 for switching the output of transmitter 25. The space diversity system adopts a system wherein a signal of a same carrier frequency is alternately transmitted and received in a predetermined period and the reception diversity and transmission diversity are operated at the base station side.
A mobile station performs control overhead and data to for the base station within a time slot to the base station, and the base station receives the overhead and the data through antennas 21 and 22 and the received information is provided to the corresponding receivers 23 and 24 and to the respective error detection circuits 29 and 30, and so forth. The base stations performs the reception space diversity control by comparing the reception bit error rate of the two reception signals through antenas 21 and 22 by the bit error comparison circuit 26 and outputs a received signal which exhibits a lower error rate through selection switch 28. Furthermore, whenever the base station transmits to the mobile station, transmission diversity control to utilize the antenna which received the lower error rate on the latest reception.
While the present system is characterized in that an antenna which is less influenced by a waveform distortion caused by frequency selective fading is selected, it is based on the presumption to employ a transmission-reception system that uses the same carrier frequency wherein the correlation between a signal from a terminal station and a signal from a base station is high and the reversibility of a transmission path is provided.
As described above, in the conventional transmission space diversity system, such control as described above is performed under the condition that the same carrier frequency is used for both reception and transmission. However, where an FDD (frequency division full duplex) communication system wherein different carrier frequencies are used for transmission and reception is adopted, or in a system wherein, while a TDD (time division duplex) communication system is adopted, carrier frequencies in transmission and reception may not necessarily be same as each other, since the manner in which multi-path fading occurs is different depending on the carrier frequencies, an effective effect of space diversity cannot be obtained by the controlling method described above.
This is described in connection with interference between a direct wave and a reflected wave when multi-path fading occurs with reference to FIG. 2. If it is assumed that the distance between a transmission point and a reception point is x=50 m, a reflection plate is located at a middle point, the angle with respect to the reflection plate is .theta.=20.degree., and the reflection coefficient of the reflecting plate is 1, the phase difference in wavelength at the reception point can be calculated from a difference in distance between the direct wave and the reflected waves dividing the distances by the wavelength. Accordingly, for example, where the transmission frequency is 20.10 GHz, the numbers of times wavelengths at the transmission and reception points are approximately 3,350.00 times wavelength with the direct wave and approximately 3,565.00 times wavelengths with the reflected wave and the difference between the wavelengths is approximately 215.00 times wavelengths. Consequently, it can be recognized that the direct wave and the reflected wave are composed substantially fully in the same phases with each other. On the other hand, where the transmission frequency is 19.96 GHz, the wavelengths at the transmission and reception points are approximately 3,326.67 times wavelengths with the direct wave and approximately 3,540.16 times wavelengths with the reflected wave, so the difference between of the distances divided by the wavelength is approximately 213.49 times wavelengths. Consequently, it can be recognized that the direct wave and the reflected wave are composed substantially fully in the inverse phases with each other. Consequently, it cannot be said that, even in fading arising from two waves, the phase of the direct wave and the reflected wave composed with a frequency difference of 140 MHz in the proximity of 20 GHz coincide with each other. Consequently, where different carrier frequencies are used for transmission and reception, improvement in communication quality against multi-path fading cannot be anticipated at all even if the space diversity system of the conventional example wherein a result of reception space diversity is used for transmission space diversity control is adopted. Therefore, where the carrier frequencies in transmission and reception are different from each other, the transmission space diversity system described above is not adopted.